Dynamic Alarm Conditions
The OPC UA client can receive alarms only if the NodeId of the alarm condition is configured in the peripheral address. This is a limitation of WinCC OA, because some servers do not export the ConditionId in the address space or even use dynamic ConditionsIds.
Such alarms are very hard to configure in WinCC OA because it is almost impossible to determine the ConditionId in advance.
For example the S7 1500 OPC UA server uses dynamic conditions and therefore WinCC OA could not receive alarms from this device up until now.
Configuration
In order to support also the reception of alarms without configuration of the ConditionId the concept of a fallback address is used. This allows that all alarms, which are received via an alarm subscription and which are not mapped to an explicit configured ConditionId are mapped to an arbitrary fallback alarm address.
This fallback address can be configured with the config entry [opcua] alarmFallbackAddress. In such a case the receiving DPE might have many active alarm instances, which is possible because the _alert_hdl used a multi-instance alarm.
If more than one condition is mapped to a DPE you should always select UA Alarm text
and UA Severity
in the subscription, because different conditions
might have difference alarm text and severity.